Ecosystem Productivity Decomposition

8 MCQs2 revision cards9-step worked example
Source: NCERT Cell Cycle and Cell DivisionPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The pyramid of energy never inverts — and NEET counts on you forgetting why.

An ecosystem's productivity starts at the producer level. Primary productivity is the total organic matter synthesised by autotrophs per unit area per unit time. Of this, Gross Primary Productivity (GPP) is the total photosynthetic output, while Net Primary Productivity (NPP) = GPP − Respiration. NPP is the biomass available to herbivores and decomposers (NCERT Class 12 Biology Chapter 13, page 292).

Decomposition breaks down dead organic matter (detritus) into simpler inorganic substances. The key steps — fragmentation, leaching, catabolism, humification, mineralisation — return nutrients to the soil. Decomposition rate depends on the chemical quality of detritus (lignin-rich detritus decomposes slowly) and climatic factors (warm, moist conditions accelerate it; low temperature and anaerobic conditions retard it).

Energy flow is unidirectional: sun → producers → herbivores → carnivores. At each transfer, roughly 10% of energy passes to the next trophic level; the rest is lost as heat through respiration (Lindeman's 10% law). This guaranteed loss means the pyramid of energy is always upright — no exceptions.

Ecological pyramids represent trophic structure graphically. The pyramid of numbers can be inverted (a single tree supports many insects). The pyramid of biomass can be inverted in aquatic ecosystems (small-bodied phytoplankton at the base, larger zooplankton above at a given instant). But the pyramid of energy is never inverted — the 10% law ensures each successive level holds less total energy.

A common mistake: claiming the pyramid of energy can be inverted just like biomass or numbers. It cannot. Energy dissipation at each level is irreversible — thermodynamics does not allow an upper trophic level to accumulate more energy than the one below it.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Which of the following statements about primary productivity is correct?

MCQ 2Easy RecallPractice

Which of the following factors retards decomposition?

MCQ 3Easy RecallPractice

The correct sequence of processes during decomposition is:

MCQ 4Direct ApplicationPractice

If the Gross Primary Productivity of an ecosystem is 20,000 kcal/m²/year and the plant respiration is 8,000 kcal/m²/year, what is the Net Primary Productivity?

MCQ 5Direct ApplicationPractice

In an ecosystem, the energy available at the producer level is 10,000 kcal. Applying Lindeman's 10% law, how much energy is available to the secondary consumers?

MCQ 6Direct ApplicationPractice

Which of the following ecological pyramids is always upright?

MCQ 7Concept TrapPractice

A student claims that in an ocean ecosystem, the pyramid of biomass is inverted, so the pyramid of energy must also be inverted. What is wrong with this reasoning?

MCQ 8CalculationPractice

In a food chain, if secondary consumers have 50 kcal of energy, how much energy was originally available at the producer level? (Assume Lindeman's 10% law applies at each trophic transfer.)

Quick recall before you leave

Worked Example

  1. 1

    Given

    A grassland ecosystem has producers with a GPP of 40,000 kcal/m²/year. Plant respiration accounts for 50% of GPP. The food chain is: Grass → Grasshopper → Frog → Snake → Hawk.

  2. 2

    Required

    Find the energy available at the tertiary consumer level (snake).

  3. 3

    Concept

    Net primary productivity is the energy available to the first consumer level. From there, Lindeman's 10% law governs each successive trophic transfer.

  4. 4

    Formula

    NPP = GPP − Respiration E_(n+1) = 0.10 × E_n

  5. 5

    Substitution

    NPP = 40,000 − (0.50 × 40,000) = 40,000 − 20,000 = 20,000 kcal/m²/year Energy at primary consumer (grasshopper) = 0.10 × 20,000 Energy at secondary consumer (frog) = 0.10 × (energy at grasshopper) Energy at tertiary consumer (snake) = 0.10 × (energy at frog)

  6. 6

    Calculation

    - Producers (NPP): 20,000 kcal/m²/year - Primary consumer (grasshopper): 0.10 × 20,000 = 2,000 kcal/m²/year - Secondary consumer (frog): 0.10 × 2,000 = 200 kcal/m²/year - Tertiary consumer (snake): 0.10 × 200 = 20 kcal/m²/year **Note on exact constants:** The factor 0.10 in Lindeman's law and the 50% respiration fraction are problem-defined exact values; they do not limit significant figures in the answer.

  7. 7

    Final answer

    Energy available at the tertiary consumer level (snake) = **20 kcal/m²/year**.

  8. 8

    Common trap

    Forgetting to subtract respiration from GPP before applying the 10% law. If you start with GPP (40,000) instead of NPP (20,000), every downstream value doubles — a systematic error that gives 40 kcal instead of 20 kcal at the snake level. Always compute NPP first. A second trap: miscounting trophic levels. Snake is the tertiary consumer (fourth trophic level), not the secondary consumer. Count: producer (T1) → grasshopper (T2, primary consumer) → frog (T3, secondary consumer) → snake (T4, tertiary consumer).

  9. 9

    Similar NEET-style question

    "In an aquatic food chain, phytoplankton fix 100,000 kcal of energy. How much energy reaches the top carnivore in a five-level food chain? Also state whether the pyramid of energy for this chain is upright or inverted, and justify your answer." ---

Before solving, remember these

Key Fact

Productivity, decomposition, energy flow

GPP > NPP (NPP = GPP − R). Energy flow: sun → producers → consumers (10% law of Lindeman). Pyramids: number, biomass, energy. Inverted pyramid of biomass in ocean (algae < zooplankton momentarily). Pyramid of energy is always upright.

-- NCERT Class 12 Biology, Ch. 13, p. 292

Formulas

Lindeman's 10% law of energy transfer

Approximately 10% of energy at one trophic level is transferred to the next; rest dissipated as heat.

SymbolQuantitySI Unit
E_nenergy at level nkcal/m²

Valid when

  • Idealised trophic transfer

Logistic population growth

Population grows exponentially at small N; growth slows as N approaches K (carrying capacity); equilibrium at N = K.

SymbolQuantitySI Unit
Npopulation size-
rintrinsic rate of increase/time
Kcarrying capacity-

Valid when

  • Verhulst-Pearl assumptions
  • Limiting resource

Species-area relationship

Number of species S in area A increases as a power law. Z (slope) = 0.1-0.2 for small areas, 0.6-1.2 for continents.

SymbolQuantitySI Unit
Sspecies number-
Aarea-
Zslope-
Cintercept-

Valid when

  • Within a biogeographic region

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Mutualism (+,+) vs commensalism (+,0) confusion

Category: Similar Terms

Mutualism benefits BOTH; commensalism benefits one, neutral for the other; parasitism +,−; predation +,−.

When it triggers

Question presents organism-pair interaction and asks for type.

How to avoid

Tabulate effects on each partner: (+,+), (+,0), (+,−), (−,−), (0,−), (0,0).

Mistake

Lists humans as r-selected (they are K-selected).

Root cause: term confusion

Correction

K-selected: large body, few offspring with high parental investment, long-lived (whales, humans, elephants). r-selected: small body, many offspring, low investment, short-lived (insects, weeds).

Mistake

Says pyramid of energy can be inverted (it's always upright).

Root cause: concept gap

Correction

Pyramid of ENERGY is ALWAYS upright (10% law guarantees decreasing energy at higher trophic levels). Pyramid of NUMBER and BIOMASS can be inverted (e.g. ocean: small algae < large zooplankton at one moment).

Past Year Questions

44 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Given below are two statements: Statement I: In ecosystem, there is unidirectional flow of energy of sun from producers to consumers. Statement II: Ecosystems are exempted from 2nd law of thermodynamics. In the light of the above statements, choose the most appropriate answer from the options given below:

1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 4(final)
NEET 2025

Given below are two statements: Statement I : The primary source of energy in an ecosystem is solar energy. Statement II : The rate of production of organic matter during photosynthesis in an ecosystem is called net primary productivity (NPP). In the light of the above statements, choose the most appropriate answer from the options given below:

1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 4(final)
NEET 2024

Tropical regions show greatest level of species richness because A. Tropical latitudes have remained relatively undisturbed for millions of years, hence more time was available for species diversification. B. Tropical environments are more seasonal. C. More solar energy is available in tropics. D. Constant environments promote niche specialization. E. Tropical environments are constant and predictable. Choose the correct answer from the options given below.

1A, C, D and E only
2A and B only
3A, B and E only
4A, B and D only
NTA Answer: Option 1(final)
NEET 2024

Which of the following statements is incorrect?

1A bio-reactor provides optimal growth conditions for achieving the desired product
2Most commonly used bio-reactors are of stirring type
3Bio-reactors are used to produce small scale bacterial cultures
4Bio-reactors have an agitator system, an oxygen delivery system foam control system
NTA Answer: Option 3(final)
NEET 2024

Given below are two statements: Statement I: Gause's competitive exclusion principle states that two closely related species competing for different resources cannot exist indefinitely. Statement II: According to Gause's principle, during competition, the inferior will be eliminated. This may be true if resources are limiting. In the light of the above statements, choose the correct answer from the options given below :

1Both Statement I and Statement II are true.
2Both Statement I and Statement II are false.
3Statement I is true but Statement II is false.
4Statement I is false but Statement II is true.
NTA Answer: Option 4(final)
NEET 2023

Identify the correct statements: A. Detrivores perform fragmentation. B. The humus is further degraded by some microbes during mineralization. C. Water soluble inorganic nutrients go down into the soil and get precipitated by a process called leaching. D. The detritus food chain begins with living organisms. E. Earthworms break down detritus into smaller particles by a process called catabolism. Choose the correct answer from the options given below:

1B, C, D only
2C, D, E only
3D, E, A only
4A, B, C only
NTA Answer: Option 4(final)
NEET 2023

Given below are two statements: Statement I : Gause’s ‘Competitive Exclusion Principle’ states that two closely related species competing for the same resources cannot co-exist indefinitely and competitively inferior one will be eliminated eventually. Statement II : In general, carnivores are more adversely affected by competition than herbivores. In the light of the above statements, choose the correct answer from the options given below:

1Both Statement I and Statement II are false.
2Statement I is correct Statement II is false.
3Statement I is incorrect but Statement II is true.
4Both Statement I and Statement II are true.
NTA Answer: Option 2(final)
NEET 2023

Given below are two statements: Statement I: Electrostatic precipitator is most widely used in thermal power plant. Statement II : Electrostatic precipitator in thermal power plant removes ionising radiations. In the light of the above statements, choose the most appropriate answer from the options given below:

1Both Statement I and Statement II are incorrect.
2Statement I is correct but Statement II is incorrect.
3Statement I is incorrect but Statement II is correct.
4Both Statement I and Statement II are correct.
NTA Answer: Option 2(final)
NEET 2023

Match List I with List II. List I List II A. Logistic growth I. Unlimited resource availability condition B. Exponential growth II. Limited resource availability condition C. Expanding age pyramid III. The percent individuals of pre-reproductive age is largest followed by reproductive and post reproductive age groups D. Stable age pyramid IV. The percent individuals of pre-reproductives and reproductive age group are same Choose the correct answer from the options given below:

1A-II, B-III, C-I, D-IV
2A-II, B-IV, C-I, D-III
3A-II, B-IV, C-III, D-I
4A-II, B-I, C-III, D-IV
NTA Answer: Option 4(final)
NEET 2022

Given below are two statements: Statement I: Decomposition is a process in which the detritus is degraded into simpler substances by microbes. Statement II: Decomposition is faster if the detritus is rich in lignin and chitin. In the light of the above statements, choose the correct answer from the options given below:

1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 4(final)
NEET 2022

Which one of the following statements is correct?

1Increased ventricular pressure causes closing of the semilunar valves.
2The atrio-ventricular node (AVN) generates an action potential to stimulate atrial contraction
3The tricuspid and the bicuspid valves open due to the pressure exerted by the simultaneous contraction of the atria
4Blood moves freely from atrium to the ventricle during joint diastole.
NTA Answer: Option 4(final)
NEET 2022

Which of the following statements is not true?

1Flippers of penguins and dolphins are a pair of homologous organs
2Analogous structures are a result of convergent evolution
3Sweet potato and potato is an example of analogy
4Homology indicates common ancestry
NTA Answer: Option 1(final)
NEET 2022

Which of the following is a correct statement?

1Mycoplasma have DNA, ribosome and cell wall.
2Cyanobacteria are a group of autotrophic organisms classified under kingdom Monera.
3Bacteria are exclusively heterotrophic organisms.
4Slime moulds are saprophytic organisms classified under Kingdom Monera.
NTA Answer: Option 2(final)
NEET 2021

Select the correct pair. 141. Which of the following statements is incorrect?

1Loose parenchyma cells - Spongy (1) Cyclic photophosphorylation involves both PS I rupturing the epidermis parenchyma and PS II and forming a lens shaped opening in bark
2Both ATP and NADPH + H+ are synthesized (2) Large colorless empty - Subsidiary cells during non-cyclic photophosphorylation cells in the epidermis
3Stroma lamellae have PS I only and lack NADP of grass leaves reductase (3) In dicot leaves, vascular - Conjunctive bundles are surrounded tissue
4Grana lamellae have both PS I and PS II by large thick-walled cells
NTA Answer: Option 1(final)
NEET 2021

In the exponential growth equation N = N ert, e t o

1(iv) (iii) (ii) (i) represents (1) The base of geometric logarithms
2(ii) (iv) (i) (iii) (2) The base of number logarithms
3(i) (ii) (iii) (iv) (3) The base of exponential logarithms
4(iii) (i) (iv) (ii) (4) The base of natural logarithms
NTA Answer: Option 4(final)
NEET 2020

‚Ÿ˜ 1987 ◊ ◊ÊÚÁã≈˛Uÿ‹ ¬˝Ê ≈UÊ ∑§ÊÚ‹ Á∑§‚ ¬⁄U ÁŸÿ òÊáÊ ∑ § Á‹∞ 101. Montreal protocol was signed in 1987 for control „SÃÊˇÊÁ⁄Uà Á∑§ÿÊ ªÿÊ ÕÊ? of :

1e-fl S≈U (e-∑ͧ«∏Ê ∑§⁄U∑§≈U) ∑§Ê ÁŸ¬≈UÊŸ (1) Disposal of e-wastes
2∞∑§ Œ ‡Ê ‚ ŒÍ‚⁄ U Œ ‡Ê ◊ •ÊŸÈfl Á‡Ê∑§Ã— M§¬Ê ÃÁ⁄Uà ¡ËflÊ (2) Transport of Genetically modified organisms ∑ § ¬Á⁄Ufl„Ÿ ∑ § Á‹∞ from one country to another
3•Ê $¡Ê Ÿ ∑§Ê ˇÊÁà ¬„È°øÊŸ flÊ‹ ¬ŒÊÕÊ Z ∑§Ê ©à‚¡¸Ÿ (3) Emission of ozone depleting substances
4„Á⁄Uà ªÎ„ ªÒ‚Ê ∑§Ê ¿UÊ «∏ŸÊ (4) Release of Green House gases H3 24 Hindi+English
NTA Answer: Option 3(final)
NEET 2020

ÉÊÊ‚ ÷ÍÁ◊ ¬ÊÁ⁄UÃãòÊ ◊ ¬Ê Ë SÃ⁄UÊ ∑ § ‚ÊÕ ¡ÊÁÃÿÊ ∑ § ‚„Ë 108. Match the trophic levels with their correct species ©ŒÊ„⁄UáÊ ∑§Ê ‚È◊ Á‹Ã ∑§ËÁ¡∞– examples in grassland ecosystem. (a) øÃÈÕ¸ ¬Ê Ë SÃ⁄U (i) ∑§ÊflÒ Ê (a) Fourth trophic level (i) Crow (b) ÁmÃËÿ ¬Ê Ë SÃ⁄U (ii) Áªh (b) Second trophic level (ii) Vulture (c) ¬˝Õ◊ ¬Ê Ë SÃ⁄U (iii) π⁄UªÊ ‡Ê (c) First trophic level (iii) Rabbit (d) ÃÎÃËÿ ¬Ê Ë SÃ⁄U (iv) ÉÊÊ‚ (d) Third trophic level (iv) Grass ‚„Ë Áfl∑§À¬ øÈÁŸ∞ — Select the correct option : (a) (b) (c) (d) (a) (b) (c) (d)

1(i) (ii) (iii) (iv) (1) (i) (ii) (iii) (iv)
2(ii) (iii) (iv) (i) (2) (ii) (iii) (iv) (i)
3(iii) (ii) (i) (iv) (3) (iii) (ii) (i) (iv)
4(iv) (iii) (ii) (i) (4) (iv) (iii) (ii) (i) H3 26 Hindi+English
NTA Answer: Option 2(final)
NEET 2020

∞∑§ ¬ÊÁ⁄UÃãòÊ ◊ ‚∑§‹ ¬˝ÊÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ •ÊÒ⁄U Ÿ ≈U ¬˝ÊÕÁ◊∑§ (4) They are useful in genetic engineering. ©à¬ÊŒ∑§ÃÊ ∑ § ‚ ’㜠◊ , ÁŸêŸÁ‹Áπà ◊ ‚ ∑§ÊÒŸ ‚Ê ∑§ÕŸ ‚„Ë 113. In relation to Gross primary productivity and Net „Ò? primary productivity of an ecosystem, which one

1‚∑§‹ ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ •Ê⁄Ò U Ÿ≈ U ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ of the following statements is correct ? ∑ § ’Ëø ∑§Ê ߸ ‚ê’㜠Ÿ„Ë „Ò– (1) There is no relationship between Gross primary productivity and Net primary
2‚∑§‹ ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ ‚ŒflÒ Ÿ≈ U ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ productivity. ‚ ∑§◊ „Ê ÃË „Ò– (2) Gross primary productivity is always less
3‚∑§‹ ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ ‚ŒflÒ Ÿ≈ U ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ than net primary productivity. ‚ •Áœ∑§ „Ê ÃË „Ò– (3) Gross primary productivity is always more than net primary productivity.
4‚∑§‹ ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ •Ê⁄Ò U Ÿ≈ U ¬Ê˝ÕÁ◊∑§ ©à¬ÊŒ∑§ÃÊ (4) Gross primary productivity and Net primary ∞∑§ „Ë „Ò •ÊÒ⁄U •Á÷ÛÊ „Ò– productivity are one and same. Hindi+English 27 H3
NTA Answer: Option 3(final)
NEET 2020

• ≈UÊ∑¸˜§Á≈U∑§ ˇÊ òÊ ◊ Á„◊-• œÃÊ Á∑§‚ ∑§Ê⁄UáÊ „Ê ÃË „Ò? temperature

1•fl⁄UÄà Á∑§⁄UáÊÊ mÊ⁄UÊ ⁄ U≈UËŸÊ ◊ ˇÊÁà (3) Inflammation of cornea due to high dose of UV-B radiation
2ÁŸêŸ Ãʬ mÊ⁄UÊ •Ê°π ◊ Œ˝fl ∑ § ¡◊Ÿ ∑ § ∑§Ê⁄UáÊ (4) High reflection of light from snow
3UV-B ÁflÁ∑§⁄UáÊ ∑§Ë ©ìÊ ◊ÊòÊÊ ∑ § ∑§Ê⁄UáÊ ∑§ÊÚÁŸ¸ÿÊ ∑§Ê ‡ÊÊÕ 118. Strobili or cones are found in : (1) Equisetum
4Á„◊ ‚ ¬˝∑§Ê‡Ê ∑§Ê ©ìÊ ¬⁄UÊfløŸ (2) Salvinia
NTA Answer: Option 3(final)
NEET 2020

Name the plant growth regulator which upon 127. ©‚ flÎÁh ÁŸÿ òÊ∑§ ∑§Ê ŸÊ◊ ’ÃÊßÿ Á¡‚ ªÛÊ ∑§Ë »§‚‹ ¬⁄U spraying on sugarcane crop, increases the length Á¿U«∏∑§Ÿ ‚ ©‚∑ § ß ∑§Ë ‹ê’Ê߸ ◊ ’…∏Ê ûÊ⁄UË „Ê ÃË „Ò, ÃÕÊ ªÛÊ of stem, thus increasing the yield of sugarcane crop. ∑ § »§‚‹ ∑§Ë ¬ÒŒÊflÊ⁄U ’…∏ÃË „Ò–

1Abscisic acid (1) ∞ é‚ËÁ‚∑§ •ê‹
2Cytokinin (2) ‚Êß≈UÊ ∑§Êߟ˟
3Gibberellin (3) Á¡’⁄ U‹ËŸ
4Ethylene (4) ∞ÁÕ‹ËŸ
NTA Answer: Option 3(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Population growth, interactions, ecosystem, biodiversity, conservation, pollution

RecallMedium

Common distractors

population interaction type confusion

Six interaction types form a 2x2 effect matrix (+,+), (+,-), (+,0), (-,-), (-,0), (0,0). Students who have memorised the symbols cannot always assign real organism pairs correctly: barnacles on a whale body = commensalism (+,0) not mutualism (+,+); mycorrhizal fungi on plant roots = mutualism not parasitism. The most common error is selecting mutualism for commensalism examples because both organisms are 'associated'.

ecological pyramid inversion error

Students learn that 'ecological pyramids can be inverted' from the examples of the number pyramid (tree-insects-birds) and the ocean biomass pyramid. They over-apply this to energy pyramids. The 10% law guarantees that energy ALWAYS decreases at each trophic level -- no realistic scenario inverts the energy pyramid. Questions ask which pyramid type 'can be inverted'; students select energy because they generalise the inversion rule.

k r selection confusion

K-selected species (large body, few offspring, high parental investment, long-lived: whales, humans, elephants) are confused with r-selected (small body, many offspring, no parental care: insects, frogs, bacteria). Students who associate 'reproducing as humans do = normal' call humans r-selected, or who associate 'complex animal = cared-for offspring' call oysters K-selected.

speciation type confusion

Sympatric speciation occurs in the same geographic area without physical isolation (e.g., polyploidy); allopatric speciation requires a geographic barrier. Students who cannot recall the Greek roots (sym = same, allo = different) confuse the two when a question describes a real-world scenario and asks which speciation type it illustrates.

Sources

NCERT refs: Class 12 Biology Chapter 13, p.292

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